1. Field of the Invention
The present invention relates to a method of and an apparatus for implementing capacity adjustment to correct variations in cell capacities.
2. Description of the Related Art
In a battery pack consisting of multiple cells, the values of the voltages at the individual cells become different from one another due to inconsistency in the levels of self discharge currents at the cells occurring over time and inconsistency in the levels of current consumption at the cell voltage monitoring circuits provided in conjunction with the individual cells. Since a constant proportional relationship is achieved between the battery capacity and the open circuit voltage in the battery pack, inconsistency in the open circuit voltage itself indicates variance in the capacities among the individual cells.
When charging a battery pack, uniformity must be achieved among the capacities of the cells by performing capacity adjustment in correspondence to the deviation in the capacity of each cell.
In a capacity adjustment method in the earlier technology, a capacity adjustment discharge circuit is provided for each cell so that uniformity is achieved among the voltages at the individual cells by using a capacity adjustment target value which is the average of the voltages at all the cells in the battery pack. Namely, while the battery pack is charged/discharged, a cell having a voltage higher than the capacity adjustment target value is discharged in correspondence to the deviation from the voltage capacity adjustment target value, so that the voltage of each cell can be set at the level of the voltage closer to the average voltage in the battery pack.
In this method, a high error criterion level and a low error criterion level each representing a specific value are set relative to the capacity adjustment target value and a decision is made with regard to whether or not there is an error at each cell by judging whether or not the voltage at the cell is beyond either error criterion level during the capacity adjustment.
However, the capacity adjustment method described above presents a problem in that if there is a cell at which a large voltage reduction relative to the voltage reductions at the remaining majority of cells has occured but is still within the normal range, an erroneous decision may be made that the cell with the relatively large voltage drop is abnormal. This problem is now explained in reference to FIGS. 15Axcx9c15C in which Cx indicates the cell with the relatively large voltage reduction. In the figures, ∘ indicates the level of the voltage at each cell, and Cx is indicated with the filled circle. FIGS. 15Axcx9c15C show changes in the cell voltages occurring over time as indicated by the arrows.
In this method, cells Ca, Cb and Cd with voltages higher than the capacity adjustment target value representing the average voltage of all the cells are discharged as shown in FIG. 15A. While the voltages at the cells except for the cell Cx are adjusted closer to the capacity adjustment target value, the voltage at the cell Cx which is lowered to a larger degree relative to the other cell voltages during the capacity adjustment becomes further deviated from the average voltage of all the cells, as illustrated in FIG. 15B. Thus, the voltage at the cell Cx becomes lower than the error criterion level set at the level lower than the capacity adjustment target value by Vd as shown in FIG. 15C, resulting in an erroneous decision that Cx manifests an abnormality.
In a battery pack provided with capacity adjustment discharge circuits, an error in the capacity adjustment function may occur in any of the cells. This error arises when there is a failure in which a capacity adjustment discharge circuit remains in an OFF state. In this situation, the value of the voltage of the cell, which has the capacity adjustment discharge circuit being in the OFF state, may change while maintaining a specific deviation from the average voltage of all the cells. Therefore, a problem may arise in that if the deviation is equal to or smaller than the Vd representing the error criterion level width, the error cannot be detected. This problem is now explained in reference to FIGS. 16Axcx9c16C. In the explanation, it is assumed that a failure has occurred in the capacity adjustment discharge circuit of a cell Cy.
The cells with voltages higher than the average voltage of all the cells representing the capacity adjustment target value alone are discharged. Namely, the cell Cy manifesting a capacity adjustment function error is not discharged although the cells Ca and Cb having voltages higher than the capacity adjustment target value are discharged as shown in FIG. 16A. By the capacity adjustment, the respective cell voltages are shifted as shown in FIG. 16B. It is assumed that the difference between the value of the voltage at the cell Cy and the average voltage of all the cells is equal to or smaller than the error criterion level width Vd when the cell voltages of the other cells excluding that of the cell Cy have been adjusted to the capacity adjustment target value as shown in FIG. 16C. In this situation, unless the voltage drop at the cell Cy has become lowered to an abnormal degree, the voltage of the cell Cy manifesting the capacity adjustment function error changes while sustaining a specific deviation from the average voltage of all the cells. This necessitates a separate detection system to be provided to make a decision as to whether or not an ON/OFF failure or the like has occurred at any of the capacity adjustment discharge circuits.
An object of the present invention is to provide a method of and an apparatus for implementing capacity adjustment on a battery pack, that prevents an erroneous decision from being made with regard to an abnormality manifesting at cells and enables a detection of a capacity adjustment function error without having to provide a separate detection system.
A battery pack for which the capacity adjustment method according to the present invention may be adopted is constructed by connecting in series multiple cells with a discharge circuit provided in conjunction with each cell. In the capacity adjustment method, the discharge process comprises a step in which open circuit voltages at individual cells are monitored over predetermined time intervals, a step in which the characteristic value of the open circuit voltages of the individual cells is set as a capacity adjustment target value, a step in which the length of the adjustment discharge time is determined for each cell based upon the capacity adjustment target value and the open circuit voltage of the cell, and a step in which the cell is discharged over the length of the adjustment discharge time thus determined. The discharge process is executed repeatedly to adjust the capacity of each cell.
A battery pack for which the capacity adjustment method according to the present invention may be adopted is constructed by connecting in series multiple cells with a discharge circuit provided in conjunction with each cell. In the capacity adjustment method, a discharge process comprises a step in which open circuit voltages at individual cells are monitored over predetermined time intervals, a step in which the value of the lowest voltage among the open circuit voltages of the individual cells is set as a capacity adjustment target value, a step in which the length of adjustment discharge time is determined for each cell based upon the capacity adjustment target value and the open circuit voltage of the cell and a step in which the cell is discharged over the length of adjustment discharge time thus determined. The discharge process is executed repeatedly to adjust the capacity of each cell.
A battery pack for which the capacity adjustment apparatus according to the present invention may be adopted contains multiple cells connected in series. The capacity adjustment apparatus comprises a monitoring circuit that monitors open circuit voltages of individual cells over predetermined time intervals, an arithmetic operation circuit that sets the characteristic value of the open circuit voltages at all the cells monitored by the monitoring circuit as a capacity adjustment target value and calculates the length of the adjustment discharge time for each cell based upon the capacity adjustment target value and the open circuit voltage of each cell, and a capacity adjustment discharge circuit that discharges the cell over the length of the adjustment discharge time calculated by the arithmetic operation circuit.
A battery pack for which the capacity adjustment apparatus according to the present invention may be adopted contains multiple cells connected in series and a means of discharging at individual cells. The capacity adjustment apparatus comprises a means of monitoring open circuit voltages of individual cells over specific time intervals, an arithmetic means of setting a characteristic value among the open circuit voltages of the individual cells monitored by said monitoring means as a capacity adjustment target value and calculating the length of the adjustment discharge time for the cells based upon the capacity adjustment target value and the open circuit voltage of the cells, and a capacity adjustment control means for discharging the cells via said discharge means over the adjustment discharge times calculated by said arithmetic means.
A battery pack for which the capacity adjustment apparatus according to the present invention may be adopted contains multiple cells which are connected in series. The capacity adjustment apparatus comprises a monitoring circuit that monitors open circuit voltages of individual cells over predetermined time intervals, an arithmetic operation circuit that sets the value of the lowest voltage among the open circuit voltages at all the cells monitored by the monitoring circuit as a capacity adjustment target value and calculates the length of the adjustment discharge time for each cell based upon the capacity adjustment target value and the open circuit voltage of each cell and a capacity adjustment discharge circuit that discharges the cell over the length of the adjustment discharge time calculated by the arithmetic operation circuit.
In addition, a battery pack for which the capacity adjustment method according to the present invention may be adopted is constituted by connecting in series multiple modules each having multiple cells connected in series with a discharge circuit provided in conjunction with each cell. In this capacity adjustment method, the discharge process comprises a step in which an open circuit voltage at each cell is monitored over predetermined time intervals, a step in which the average of the cell voltages in the module with the lowest module voltage is set as a capacity adjustment target value, a step in which the length of the adjustment discharge time is determined for a given cell based upon the capacity adjustment target value and the open circuit voltage of the cell and a step in which the cell is discharged over the length of the adjustment discharge time thus determined. The discharge process is executed repeatedly to adjust a capacity of each cell.
A battery pack for which the capacity adjustment apparatus according to the present invention may be adopted includes multiple modules connected in series, each constituted by connecting in series multiple cells. The capacity adjustment apparatus comprises a monitoring circuit that monitors open circuit voltages of individual cells over predetermined time intervals, an arithmetic operation circuit that sets the average of the cell voltages in the module with the lowest module voltage as a capacity adjustment target value and calculates the length of adjustment discharge time for each cell based upon the capacity adjustment target value and the open circuit voltage of the cell and a capacity adjustment discharge circuit that discharges the cell over the length of the adjustment discharge time.